Yesterday, in What are Good Sources of Protein? – Digestion Speed Part 1, I looked briefly at the issue of protein turnover and synthesis and then looked even more briefly at the now infamous Boirie study that kicked off the interest in fast and slow proteins.
Summing up, that study found that whey and casein (the two proteins found in milk) digested at different speeds, with whey being a ‘fast’ protein that spiked amino acid levels before dropping (after 3-4 hours), and casein being a ‘slow’ protein that raised amino acid levels more gradually but remaining stable for an extended period (7-8 hours).
Of more relevance, the researchers also found that the whey protein stimulated whole-body protein synthesis without much effect on protein breakdown while casein decreased protein breakdown with little effect on protein synthesis; I’d note that there was also an increase in the oxidation (burning for energy) of the whey protein. Thus whey became known as an ‘anabolic’ protein and casein an ‘anti-catabolic’ protein.
More Commentary About the Boirie Study
One other note that I didn’t mention yesterday. The researchers also looked at how each protein impacted on net leucine balance, that is how much leucine was actually stored in the body (this is used as an indicator of what’s going on with other amino acid levels).
Despite the fact that the whey actually stimulated more protein synthesis, the casein had the larger impact on leucine balance; at the end of the feeding period, the body had stored more leucine with the casein. Phrased a bit differently, it looked as if decreasing protein breakdown was more important than increasing protein synthesis in terms of whole body leucine (and therefore, protein balance).
This study essentially created an entirely new industry in the world of sports nutrition. Interestingly (or amusingly depending on your perspective), the study was interpreted variously depending on whether the company in question was selling whey or casein. Companies selling whey focused on the increase in protein synthesis; those selling casein either pointed to the increased oxidation of whey or the fact that casein had a greater impact on net leucine balance.
Various practical suggestion came out of this as well at least in the world of sports nutrition, whey protein was suggested for first thing in the morning to get aminos into the bloodstream as quickly as possible. I’d note again that this isn’t exactly the case and I realize that this is a little bit confusing. As shown in Figure of in What Are Good Sources of Protein? – Speed of Digestion Part 1 both casein and whey start to appear in the bloodstream at about the same time point; however, whey certainly raises blood amino acid levels more quickly at that point. In contrast, casein was suggested for bedtime to provide aminos throughout the fasting period to stave off muscle breakdown.
Almost without exception, whey was suggested as the best protein for after training to get aminos into the bloodstream more quickly. As noted yesterday, not only is this not true but there is emerging data (discussed in detail in The Protein Book) that fast proteins after training are not the optimal choice for promoting lean body mass gains, slow proteins or a combination of slow and fast proteins appear to be more effective. I’d refer readers back to my article on Milk: The New Sports Drink? A Review where milk outperformed soy (a fast protein) for promoting lean body mass gains.
Others suggested that the combination of whey and casein should be superior to either in isolation; the whey provides a quick hit of aminos to boost protein synthesis while the casein provided a longer source of aminos to blunt protein breakdown. In many ways, this third group would turn out to be closer to correct than either the whey-only or casein-only groups. But I’m getting ahead of myself.
But Wait, There’s More
As I also noted yesterday, one limitation of the study in question was that the protein was given without any other nutrients (carbs or fats) and were given to folks who had fasted overnight; of course it didn’t involve any type of training (which can change the dynamics of how protein is used in the body).
Frankly, the extrapolations being made about whey or casein from the study in question were poor for this (and other reasons).
Of course, later research ended up addressing some of these issues. A followup study titled “Influence of the protein digestion rate on protein turnover in young and elderly subjects.” looked at the impact of whey and casein when it was combined with carbohydrates and fats. And the differences between the two basically disappeared under these conditions.
While whey still got aminos into the bloodstream a touch more quickly, the casein meal still had the edge in terms of net leucine retention. It’s important to note that, in both of the original studies, the amount of protein given in the whey and casein group weren’t identical; the casein group got a bit more protein and that alone might have explained the greater gain in protein.
A third study provided identical amounts of casein and whey in mixed meals to either young or elderly folks; in that study the whey group came out a bit ahead in the young but way ahead in the older subjects (older here means 72 years old). This suggested that, even in the context of mixed meals, whey had a slight edge.
I’d note here that emerging research shows that older individuals respond very different to protein than younger; their muscles appear to become insensitive to protein to some degree and various interventions (such as protein pulse feeding or fast proteins) which spike blood amino acids appear to be vastly superior. In younger individuals, this doesn’t appear to be as significantly the case.
In any case, the data on the topic was clearly pretty mixed and, in the context of a mixed meal (which is how most people eat), while whey might have a slight edge over casein it was small at best.
I’d note that none of the above applies to nutrition around training, which is a topic that I’ll have to cover in a separate article. Training changes the dynamics of a many things including how protein is used by the body so the data discussed so far doesn’t really apply to that specific situation.
The Impact of Previous Meals on Digestion Speed
Finally, before moving on, a final topic that I’ve mentioned above, which is the fact that most of the studies done feed the protein to folks after an overnight fast. While this makes the studies less complicated, it doesn’t indicate what happens when a meal is being consumed while another is still digesting.
Unfortunately, this area is poorly studied, I’m not aware of any work that has examined if the fast/slow protein concept has any relevance to a meal being eaten later in the day. I’d only note again that whole food meals take much longer to digest than is often claimed, a moderate sized meal may still be digesting 5-6 hours later. Even if a ‘fast’ protein is consumed, there’s no guarantee it will still act ‘fast’ if there’s still food sitting in the gut. Again, I’m unaware of any research on this topic.
What About Other Proteins?
So far all I’ve focused on is whey and casein as these are the proteins that have been predominantly studied. Unfortunately, there is far less data on the speed of digestion of other proteins. Soy has been examined and appears to be a fast protein; note from my article Milk: The New Sports Drink? A Review that milk was found to be superior to soy protein for gains in lean body mass with training. This is thought to be due to the rapid digestion and amino acid profile of soy.
Beyond that not a tremendous amount of data exists. One researcher collected what is available and I’ve reproduced his data (originally printed in The Protein Book) in the table below
|Protein||Absorption Rate (g/hour)|
|Raw Egg Protein *||1.4|
|Cooked Egg Protein *||2.9|
|Soy Protein Isolate||3.9|
|Tenderloin Pork Steak *||10.0|
* Measurements marked with an asterisk should be considered as the roughest estimates as the studies used indirect measurements of protein digestion.
Clearly there is a large variety for protein digestion rates although, as noted, some of the above values should be taken as very rough estimates.
I’d note again that this has some implication for the idea that you must eat protein every three hours. With the exception of whey, where 40 grams of protein would take roughly 4 hours for complete absorption), all proteins listed would still be digesting for far longer than the magic 3 hour period.
Once again, this is getting a bit too long so I’m going to save the final bit of this discussion for Part 3, which I’ll post on Monday. In that article I’ll look at the current fascination in the sports nutrition industry with protein hydrolysates along with a few minor topics.